JPS623691A - fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to fuel assemblies for nuclear reactors, and more particularly to flexure resistant fuel assembly structures for use in non-control rod locations of a reactor core.

The core of an unlucky EΔI-absorbing nuclear reactor typically includes a plurality of fuel assemblies. In a typical pressurized watercolor reactor (PWR), all fuel assemblies are similar in geometry. Each fuel assembly includes a plurality of fuel rods held in an organized array by a grid spaced along its length. The grid is simply mounted to guide multiple control rods.

The upper and lower nozzles of the fuel assembly are secured to opposite ends of a control rod guide simple that extends above and below the upper and lower ends of the fuel rods, respectively. The control rod guide simple and the upper and lower nozzles securely attached thereto together constitute the framework structure of the fuel assembly.

To control the fission process produced by the nuclear fuel contained within the fuel rods, a number of control rods are typically reciprocably disposed within the guide simple of the fuel assembly. However, not all fuel assembly locations in the core use control rods; only about one-third of fuel assemblies are located at control rod locations; Since the bodies were constructed to be similar in geometry, this means that the fuel assemblies at the control rod locations are the same as the fuel assemblies at the non-control rod locations.

An alternative approach to this conventional practice of constructing all PWR fuel assemblies in the same way has recently been proposed. Another construction of a fuel assembly for use in a non-control rod location includes a lower nozzle, a number of longitudinally extending structural members containing burnable poison, and an upper nozzle. The fuel assembly also includes a number of axially spaced grids secured to the longitudinal structural members and supporting an array of fuel rods. The upper and lower nozzles are secured to the longitudinal structural members by threaded connections or other suitable rigid attachment means.

An instrumentation tube is centrally located in the fuel assembly and supported by upper and lower nozzles and grids.

One important difference of this non-control rod location fuel assembly to conventional control rod location fuel assemblies is the longitudinal structural members that interconnect the upper and lower nozzles to form the framework structure of the fuel assembly. It is in the structure of In conventional PWR fuel assemblies, the structural members are hollow guiding simple tubes that are open at the top and closed (except for small holes for coolant flow) at the bottom.

These simple guide tubes are designed to align with the control rods.
It is located within the l aggregate. During operation of a nuclear reactor, control rods are reciprocated within these guided simple tubes. On the other hand, in non-control rod fuel assemblies intended for use in non-control rod core locations, the structural member, also in the form of a tube, does not house control rods, and therefore a different Functional and structural uses are possible.

8! Potentially, this non-control rod structural member contains combustible absorbent material. Combustible absorbers, such as suitable compounds of boron, are used in modern nuclear reactors as an additional means to control reactivity, especially during the early stages of the nuclear fuel's life. Architecturally, the elongated tube of the structural member is closed at each end by an end plug welded to the tube. Preferably, the tube and end plug material is Zircaloy-4; the spring holds the combustible absorbent material in place within the tube to accumulate the helium gas released when neutrons act on the boron atoms. Build a plenum for the purpose. The tube must be empty and open at one end in order to incorporate non-control rod structural members into the fuel assembly. After the grid is bulged and secured to the tube, the combustible absorbent material and spring are loaded into the tube and the remaining end plug is welded in place. Thereafter, the fuel rods are loaded and the upper and lower nozzles are bolted on.

There are eight absorber structures in non-control rod fuel assemblies, while conventional control rod fuel assemblies have 24 guide simples. Therefore, there are 16 extra fuel rods in each non-control rod fuel assembly, which provides the advantages described in Japanese Patent Application No. 61-105042 mentioned above.

The use of non-control rod fuel assemblies in PWRs having the structure described above has long been a concern, and has an important problem that affects the overall properties of PWR fuel assemblies, namely fuel assembly warping or A potential need has arisen to overcome the problem of curvature. There appears to be a clear relationship between the degree of curvature of the fuel assembly and the compressive stress in the guiding simple. Unfortunately, there is no obvious way to significantly reduce the compressive stresses in the control rod fuel assembly guide simples present at the control rod core location. However, for fuel assemblies located in non-control rod core locations and designed as described above, the compressive stresses in the longitudinal structural members are large. There appears to be a need to find ways to alleviate this.

Hikari, LL The present invention provides an improved longitudinal structural member for a non-control rod fuel assembly designed to meet the above-mentioned needs. In the above-mentioned non-control rod fuel assembly, which is disclosed in detail in the above-cited Japanese Patent Application No. 105042/1982, the axial force from the upper nozzle compression spring is transmitted vertically from the upper nozzle adapter plate. It is transmitted to the upper end plug of the directional structure member, through its cladding tube to the lower end plug, and from there to the adapter plate of the lower nozzle. The cladding is therefore placed in a compressed state that does not cause permanent warping of the fuel assembly.

The present invention improves the construction of longitudinal structural members to significantly reduce or offset the undesirable effects of compressive stresses on the longitudinal structural member cladding. Basically, as a solution, the cladding of the structural member is preloaded in tension. Preloading the cladding of a free-standing structural member of this type in tension means that a compressive load must be applied to the central part of the structural member. Also, the material to which the compressive load is applied must not be subject to creep caused by heat or irradiation.

Otherwise, the structural member will permanently creep into the warped position. Therefore, the central part is made of material with creep resistance.
Ceramic materials with high creep resistance, which must be formed from ceramic materials, can be used. Accordingly, the present invention contemplates providing a unique configuration for applying compressive loads to ceramic materials, preferably in stacked pellet form, thereby preloading cladding tubes into tension.

Accordingly, the present invention is directed to fuel assemblies used in non-control rod locations in nuclear reactor cores. The fuel assembly has upper and lower nozzles and a plurality of longitudinal structural members extending between and attached to the nozzles to form the fuel assembly into an integral unit structure. At least some of the structural members include an elongated hollow cladding tube extending between the upper nozzle and the lower nozzle, and a cladding tube attached to both ends of the cladding tube to hermetically seal and cover the cladding tube. and sealing and attachment means for attaching the tube to the upper and lower nozzles.According to a refinement of the invention, (a) a quantity of material disposed within said cladding tube and having resistance to creep caused by irradiation; , (b) applying a predetermined compressive load to the creep resistant material disposed within the cladding and, as a reaction to the load, an axial load typically transmitted through the unit structure of the fuel assembly. Pretensioning means are provided for significantly reducing compressive stresses in the cladding of the structural member by preloading the tube to a pretensioned state having a magnitude sufficient to substantially offset the pretensioning. It will be done.

In particular, the creep resistant material is a ceramic material such as zirconium oxide in pellet form. The ceramic pellets are coated with a combustible absorbent material. Also, the pretensioning means may be in either of two embodiments.

In one embodiment, the pretensioning means is an elongated bellows-type device disposed within the cladding between the stack of creep-resistant pellets and one tube end. pressurized, thereby generating a predetermined axial force within the bellows-type device, which axial force places the creep-resistant pellet under compression;
The cladding is placed under pretensioning means. Additionally, the remaining portion of the cladding tube can be pressurized. In another embodiment, the pre-tensioning means is an array of disc springs disposed within the cladding tube between the stack of creep resistant pellets and one end of the cladding tube. According to this configuration, a predetermined axial force is generated in the cladding tube, which forces the creep-resistant material into compression, and sets the cladding tube in a pretensioned state. Stacked in parallel and series in an array.

The above-mentioned and other advantages and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description in conjunction with the drawings illustrating embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the following description, the same reference symbols refer to the same or corresponding parts or elements throughout the drawings. In addition, in the following explanation, "front", "back", "left"
, "rightward,""upwards or upwards,""downwards or downwards," and similar terminology are merely words of convenience and are not to be construed in a limiting sense.

11 and 1. Referring to the drawings, and in particular to FIG. 1, there is shown an uncontrolled portion of a nuclear reactor core (not shown) shown in vertically shortened form and generally designated by the reference numeral 10. An elevational view of a fuel assembly adapted for use in a rod position is shown. Basically, the fuel assembly 10 includes a lower end structure, ie, a lower nozzle 12, for supporting the fuel assembly on a lower core plate (not shown) in the reactor core, and a lower end attached to the lower nozzle 12. and a plurality of longitudinally extending structural members 14 projecting upwardly from the lower nozzle 12. The fuel assembly 10 also includes a plurality of horizontal grids 16 spaced apart along the structural member 14 in the axial direction, and an organization of elongated fuel rods 18 spaced apart laterally and supported by the grids 16. It has an array. Furthermore, the fuel assembly 10 has an instrumentation tube 20 disposed at its center and an upper end structure, or upper nozzle 22, attached to the upper end of the structural member 14. With this arrangement of the parts, the fuel assembly 10 forms an integral unit structure that can be handled appropriately without damaging the parts.

As mentioned above, the rows of fuel rods 18 in the fuel assembly 10 are held in spaced apart relation to each other by grids 16 that are axially spaced along the length of the fuel assembly. Each fuel rod 18 has a nuclear fuel pellet 24 closed at both ends by an upper end plug 26 and a lower end plug 28 . A fuel pellet 24 of fissile material generates the reaction power of the reactor. Liquid moderators and coolants, such as water or boron-containing water, are used to generate useful work.
It is pumped upwardly along the fuel rods 18 of the fuel assembly 10 for the purpose of extracting the heat generated in the fuel assembly 10.

? 2 and 3, each of which depicts a longitudinal structural member according to the present invention that can be used in a fuel assembly 10. Two slightly different embodiments of 14 are shown. As mentioned at the outset, the fuel assembly 10 is designed to be used in a non-control rod core location, so there are no control rods operatively associated with this fuel assembly.

The control rod guide of the fuel assembly at the control rod core location receives the control rods for reciprocating movement downward through its upper end and receives the coolant flow upward through its lower end. In contrast, the longitudinal structural members 14 in the non-control rod fuel assembly 10 are sealed at both ends.

Each embodiment of the longitudinal structural member 14 shown in FIGS. 2 and 3 includes an upper end plug 32 and a lower end plug 3 welded to the tube.
It has an elongated hollow cladding tube 30 closed by 4. The material of the cladding tube and end plug is preferably Xirocaloy-4. Upper end plug 32 and lower end plug 34 have axially extending threaded studs 38 and 40, respectively, secured thereto, which studs are connected to respective adapters of upper nozzle 22 and lower nozzle 12. It is inserted into the openings in plates 42 and 44. Threaded studs 38 and 40 are tightened with nuts 46 and 48, respectively, to connect opposite ends of structural member 14 to upper nozzle 22, respectively.
and firmly fixed to the lower nozzle 12. As a typical example,
The cladding tubes 30 of the structural members 14 have a substantially greater thickness than the tubes of the control rod guide simples, and the number of structural members 14 in each non-control rod fuel assembly 10 considerably less than the number of guiding simples in the body,
The remaining positions in the non-control rod fuel assembly 10 corresponding to the positions occupied by the guide simples in the control rod fuel assembly are occupied by fuel rods 18. For example, as a typical example, the control rod fuel assembly has While there are 24 guide simples, there are 24 guide simples in the non-control rod fuel assembly 10.
, there are only eight structural members 14, so the remaining 16 positions are occupied by additional fuel rods. Upper end plug 32, lower end plug 34, stud 38
．． 40, and nuts 1 to 46 and 48 constitute sealing attachment means for the cladding tube 30.

An improvement in the structural member 14 of the present invention over that disclosed in the above-cited Japanese Patent Application No. 61-105042 consists in configuring the structural member 14 to resist warping or curving. .

Each structural member 14 is rendered resistant to warping by pre-tensioning its cladding 30. In order to apply tension to the cladding tube 30 in advance in this way,
The compressively loaded material, which must be compressively loaded in its central part, must not be subject to thermal or irradiation creep.

Otherwise, it would creep into a permanent warp condition. Accordingly, the cladding 30 of the structural member 14 contains a material 50, preferably in the form of pellets and stacked within the cladding 30, that is resistant to creep due to heat or radiation (creep-resistant material). There is.

Ceramic materials such as zirconium oxide or alumina are typical examples of materials that have very high resistance to creep. Additionally, these materials can be coated with a combustible absorbent material such as boron carbide, thereby performing the same function as the structural members disclosed in Japanese Patent Application No. 105042/1983 cited above. It can also be done.

In order to apply a compressive load to the ceramic bellet 1-50 and thereby pretension the cladding 30 in response to the load, two of the pretensioning means shown in FIGS. 2 and 3 are used. Any of the embodiments can be adopted.

In FIG. 2, the pretensioning means is a bellows type device, indicated generally by the reference numeral 52, and is located within the cladding tube 30 in the upper plenum region of the structural member 14. This bellows type device 52 is connected to the upper end plug 32 by welding or the like. The outer diameter of the bellows device 52 is radially supported by the cladding tube 30 and the lower end 5 of the bellows device 52
4 is pressed against the stack of pellets 50. A pressurizing passage 56 on the side of the upper end plug 32 allows the interior of the bellows-type device 52 to be pressurized. After pressurization, the passage 56 is sealed. At such times, there is an axial force within the bellows device 52;
This force compresses the stack of pellets 1-50 and the cladding tube 30
is pretensioned.

At a bellows pressure of about 600 ps i (cold), an axial force of about 120 bonds is generated during wet operating conditions. Note that zero axial stress is not required to reduce fuel assembly warpage to an acceptable degree, and therefore it is possible to use lower pressures than the above values. If desired, the interior of the cladding tube 30 in the pellet area may be provided with a passageway 58 formed in the lower end plug 34.
Pressure can be applied via. It should be noted that when pressurizing in this manner, the pressure within the bellows cancels out and the bellows pressure must be increased accordingly.

Alternatively, the pretensioning means is in the form of an array of disc springs 60, as shown in FIG. The disc springs 60 are arranged in parallel as shown to achieve a high spring rate, and
They can be stacked in pairs as shown to obtain a large range of deflection.

Regardless of which embodiment of pretensioning means is adopted,
Its function is to apply a predetermined compressive load to the creep resistant material 50 within the structural member 14 and preload the cladding tube 30 to a pretensioned state by the reaction of this load. This pretension is applied to the fuel assembly 1 installed in the reactor.
It should be large enough to substantially offset the axial loads normally transmitted through the unitary unit structure of the structural member 14, thereby significantly reducing compressive stresses in the cladding 30 of the structural member 14. A compressive load is applied to the stack of combustible absorbent bellets 1 to 1, although the load is smaller than that of conventionally used coil springs. However, the amount of force available from a coiled spring fitted inside the cladding is much smaller than the amount of force required to create a significant pretension in the cladding, and therefore The cladding pretension provided was incidental and insufficient. For example, a 17x17 fuel assembly has a spring force of approximately 953 bonds at the beginning of its wet life.

Since there are eight structural members 14, each structural member has approximately 1
He will be responsible for 20 Bonds. In order to achieve a cladding that is stress-free during operation, a pre-tension load of 120 bonds is required for each structural member 14. This is much higher than the pre-tension load that can be achieved with a coil spring fitted inside the cladding tube.

According to the present invention, when the compressive force of the core plate is applied to the compression spring 62 of the fuel assembly 10, the cladding tube 30 of the structural member ]4
The compressive stress in the axial direction is greatly reduced. This is illustrated in FIG. FIG. 4 is a graph showing the load-stress curve of the structural member structure of Japanese Patent Application No. 105042/1983 cited above in comparison with the structural member structure of the present invention (all coating and diameter parameters are It is clear from this graph that the pre-tensioned cladding of the structural member of the present invention, for a given axial load,
It has a much lower compressive stress than the non-pretensioned tube of '42.

The idea of the invention and its many advantages will be understood from the above description, without departing from the scope of the invention or sacrificing its substantial advantages.
Obviously, many modifications may be made to the embodiments, structure and arrangement of the invention, and it is to be understood that the embodiments set forth herein are merely preferred or exemplary embodiments.

[Brief explanation of the drawing]

FIG. 1 is an elevational view of a non-control rod fuel assembly with longitudinal structural members constructed in accordance with the present invention, with a portion cut away for clarity of illustration and partially vomited; FIG. The figure is the first
FIG. 2 is an enlarged partial vertical cross-sectional view of the fuel assembly of FIG. 1, one of the longitudinal structural members attached at both ends to the adapter plates of the upper and lower nozzles of the fuel assembly and means for pretensioning the structural member; FIG. 3 is a diagram showing an example of
FIG. 4 is an enlarged partial vertical cross-sectional view of the fuel assembly of FIG. 1 similar to FIG. 2 illustrating another embodiment of the means for pretensioning the structural members; FIG. Comparing the longitudinal structural member and the longitudinal structural member disclosed in Japanese Patent Application No. 105042/1988 cited herein, it was found that the pretensioned structural member of the present invention is better than the structural member disclosed in Japanese Patent Application No. 1986
105042 is a load-stress curve diagram to illustrate that the structural member has a much lower compressive stress for a given axial load than the non-pretensioned structural member disclosed in No. 105042. 10...Fuel assembly 12...Lower nozzle 14
... Structural member 22 ... Upper nozzle 30 ...
- Cladding tube 50... Creep resistant material (bellet) pre-tensioning means 52... Bellows type device 60... Sealing/attachment means for disc spring cladding tube 32... Upper end plug 34... Lower end Plug 38・
... Stud 40... Stud 46... Nut 48... Nut...

Claims (1)

Claims: an upper nozzle, a lower nozzle, and a longitudinal direction extending between and attached to the upper and lower nozzles to form the fuel assembly into an integral unit structure. a plurality of structural members, at least some of which include an elongated hollow cladding tube extending between the upper nozzle and the lower nozzle; and a plurality of structural members fixed to both ends of the cladding tube. In a fuel assembly used in a non-control rod position of a nuclear reactor core, the fuel assembly includes a sealing/attachment means for airtightly sealing a cladding tube and attaching the cladding tube to the upper nozzle and the lower nozzle. a) a quantity of creep-resistant material disposed within the cladding tube and having resistance to creep caused by irradiation; and (b) a quantity of creep-resistant material disposed within the cladding tube to apply a predetermined compressive load to the creep-resistant material. , such that as a reaction to the compressive load, there is a pretension condition of sufficient magnitude to substantially offset the axial load typically transmitted through the unit structure of the fuel assembly. A fuel assembly for use in a non-control rod position of a nuclear reactor core improved to include: pre-tensioning means for significantly reducing compressive stress in the cladding of the structural member by applying a preload to the cladding.